CD26 also known as DPP4 is a membrane glycoprotein and serine exopeptidase that plays a crucial role in immune regulation by cleaving X-proline dipeptides from the N-terminus of polypeptides. CD26 is primarily located on the surface of T cells, where CD26 functions as a co-stimulatory molecule that enhances T cell activation and regulates chemokine activity. This localization is vital for the immune response, as CD26 interacts with various proteins, including CXCR4 and gp120, which are implicated in the pathophysiology of HIV infection. CD26′s interaction with adenosine deaminase on dendritic cells facilitates a costimulatory signal in the immunological synapse, promoting the production of proinflammatory cytokines such as interferon-gamma, tumor necrosis factor-alpha, and interleukin-6. The interaction between CD26 and caveolin-1 contributes to the up-regulation of CD86 on monocytes, which is essential for effective antigen-specific T cell activation.